[mips] Fix li/la differences between IAS and GAS.

Summary:
- Signed 16-bit should have priority over unsigned.
- For la, unsigned 16-bit must use ori+addu rather than directly use ori.
- Correct tests on 32-bit immediates with 64-bit predicates by
  sign-extending the immediate beforehand. For example, isInt<16>(0xffff8000)
  should be true and use addiu.

Also split li/la testing into separate files due to their size.

Reviewers: vkalintiris

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D10967

llvm-svn: 242139